1. Field of the Invention
The present invention relates to a method of molding a low melting point metal alloy such as a magnesium alloy, an aluminum alloy or the like using a metallic raw material, which exhibits thixotropy properties in a solid-phase and liquid-phase coexisting temperature region.
2. Description of the Related Art
A method of molding a magnesium alloy comprises the steps of melting a metallic raw material into a liquid alloy at a liquidus temperature or higher, causing the obtained liquid alloy to flow downward on a surface of an inclined cooling plate to cool the alloy rapidly in a semi-molten metal state, holding the semi-molten metal alloy in a storage tank at a temperature in a solid-phase and liquid-phase coexisting temperature region to form a metal slurry (semisolid) having thixotropy properties, casting the metal slurry to a metallic raw material potentially having thixotropy, heating this metallic raw material in a semi-molten metal state with an injection device, and injecting the heated metallic raw material into a mold to mold the material into an article while accumulating the heated metallic raw material.
Further as a molding means for a magnesium alloy or the like, a means is known that it includes a heating means on an outer circumference of a cylinder body having a nozzle opening at the end, and supplies a metallic material in a thixotropy state to a molten metal holding cylinder (heating holding cylinder) in an end portion of which a measuring chamber connected to the nozzle opening is formed with diameter reduced while being accumulated therein, and then injects the metallic material into a mold after measuring the metallic material by forward and backward movements of an internal injection plunger.
The above-mentioned related arts are disclosed in Japanese Laid-Open Patent Publications No. 2001-252759 and No. 2003-200249.
A semisolid material, which exhibits thixotropy properties in a solid-phase and liquid-phase coexisting temperature region, has a fluidity of a low viscosity by coexistence of a liquid phase and finely spheroid solid phase. This semisolid material is heated at a temperature in a solid-phase and liquid-phase coexisting temperature region because thixotropy properties must be kept until the material is injected. Since the solid phase is grows with the lapse of time even at a temperature in the solid-phase and liquid-phase coexisting temperature region, a solid-phase fraction is increased with the lapse of time and the density of the solid phase is increased so that the fluidity is lowered. Therefore, the injection of accumulated semisolid material is preferably carried out within allowable time.
When such a semisolid material is kept at a temperature in a solid-phase and liquid-phase coexisting temperature range and the molding of the material is temporarily suspended while leaving the material in a heating holding cylinder as it is, the fluidity of the semisolid material is lowered by growth of a solid phase during the suspension and it becomes difficult to perform injection by resuming the molding. If the suspension time is within allowable time the injection can be continued. However, if the suspension time is prolonged, the viscosity of the material is increased by largely grown solid phase and a flow resistance is increased whereby smooth injection cannot be made. The largely grown solid phase can be a cause of scuffing to an injection plunger or clogging or the like and the molding of the material cannot be performed.
When the molding operation of such a semisolid material is finished without discharging the remaining material at the end of molding, the solid phase continues to grow until the semisolid material reaches a solidus temperature whereby the semisolid material becomes a solid. Even if the solid is again heated to the temperature in the solid-phase and liquid-phase coexisting temperature region to be in a semi-molten metal state, since a once grown solid phase is not changed into a small solid, the solid does not return to an original semisolid material, which exhibits thixotropy properties whereby it becomes a semisolid material, which has a high viscosity and an extremely low fluidity. Thus the injection of the semisolid material as it stands becomes impossible.
The remaining semisolid material can be solved by repeating injection operation to discharge the material at the end of molding. However, even if the injection operation for the remaining semisolid material is repeated in a semisolid state, a part of the material often adheres to and remains on an inner wall surface of the heating holding cylinder, the injection plunger or the like. This adhered material is not melted at a temperature in the solid-phase and liquid-phase coexisting temperature region. Thus, when a new material is supplied without removing an adhered material and a molding operation of the material is started, the adhered material causes scuffing, clogging or the like in the injection plunger. Accordingly, the heating holding cylinder must be heated to a liquidus temperature or higher to melt and discharge the adhered material before the starting of molding.